In order to improve sealing performance, it is standard practice to provide a sealing gasket in containers between surfaces that are to seal against one another. For instance a gasket may be provided in the double seam at both ends of a fixed head drum or a gasket may be lined on to a can end that is to be used for closing a can. These sealing gaskets are conventionally deposited from liquid sealing compositions that are lined on to closure or other surface that is to carry the gasket and are dried on the surface, optionally with heating. The gasket must adhere well to the surfaces between which it is to provide a seal, must be cohesive, and must be sufficiently elastomeric to provide a good seal. The composition always includes elastomeric material, and optionally filler, and the choice of elastomeric material greatly influences the performance of the gasket.
A very large number of polymeric materials have been proposed in the literature for use as elastomeric material. One that is very widely used (especially in latex-based compositions) is styrene butadiene, that is often carboxylated as a result of the inclusion of a small amount of carboxylic monomer. This can give good results when the container is to be filled with a conventional aqueous composition, such as fruit, vegetables or beverages. However it is less satisfactory when the composition is to contain fat or oil, for instance fatty meat, chicken soup or product containing vegetable oil.
We have described in EP 182674 that, when the gasket is to be used for such a filling, improved results are obtained if the elastomeric material comprises a copolymer of styrene and an acrylic ester. The exemplified polymer is formed from 3% acrylic acid, 47% styrene and 50% butyl acrylate. A comparison polymer was formed from 2.5% methacrylic acid, 51% styrene, 26.5% butadiene and 20% butyl acrylate. These two polymers were subjected to two series of tests with and without cross linking with a vulcanising agent. In one series their sealing performance was evaluated both with a conventional aqueous filling and with a filling containing 2% vegetable oil. In another test, an open oil immersion test is conducted to determine the effect on a gasket of the gasket being immersed in olive oil at 90.degree. C. for two hours. In each instance, the comparative styrene-butadiene-acrylic ester polymer was found to be inferior to the styrene-acrylic ester polymer. Vulcanisation was shown to be relatively insignificant.
Although the styrene-acrylate copolymers can be used successfully when the container is to be filled with a filling containing an edible fat or oil, they have proved unsuccessful when the filling is to include an industrial organic solvent, for instance mineral oil or toluene or other hydrocarbon based solvent of the type that is included in, for instance, paint thinners.
In order to achieve satisfactory results, the copolymers must have good adhesion, cohesion and chemical resistance and if even one of these properties is weak then the copolymer will be unsuccessful as a sealing composition for containers holding organic solvents as described above.